Gliese 581 c: Chances for Habitability

by Paul Gilster on April 25, 2007

Just what might we find on Gliese 581 c, the potentially habitable planet announced yesterday? Much depends on where the planet formed in its circumstellar system. For that kind of information I listen to Greg Laughlin (University of California at Santa Cruz), whose work on planetary formation via core accretion seems to gain stature with every new planetary find. Here’s Laughlin’s take from his systemic weblog:

The planet probably migrated inward to its current location from beyond the “snowline” in GL 581’s protostellar disk, and so its composition likely includes a deep ocean, probably containing more than an Earth’s mass worth of water. Atmospheric water vapor is an excellent greenhouse gas, so the conditions at the planet’s atmosphere-ocean boundary are probably pretty steamy. It’s also possible, however, that the planet formed more or less in-situ. If this is the case, it would be made from iron and silicates and would be fairly dry. It’s unlikely, but not outside the realm of possibility, that this could be a genuinely habitable world. There’s no other exoplanet for which one can make this claim. In short, it’s a landmark detection.

Remember that the orbital period for Gliese 581 c has been determined to be 12.9 days, putting it in the heart of the star’s habitable zone. Laughlin’s systemic project has reason to celebrate this morning as we continue to digest the recent developments. The systemic collaboration is a publicly available simulation that models planetary systems using radial velocity techniques, with data for each star studied made available over the Net. Six of systemic’s users had already created models for Gliese 581 that jibe with the recently announced discovery, a testimony to the power of systemic and of collaborative science on widely distributed computers.

And let’s not forget the larger trends the new planetary find highlights. The discovery paper (citation in the previous post) notes that small planets — Neptune mass and below — are more frequent than gas giants around M dwarfs. We have six very low mass detections as against three Jovian planets. “This result was significant,” the paper says, “at the 97 % level before the detection of the two new Gl 581 planets…even without accounting for the poorer detection efficiency for lower-mass planets.” We looked at those trends in a recent Centauri Dreamspost.

On the frequency of detections, let me quote the paper at greater length:

The fraction of detected Neptune (and lower-mass) planets around M dwarfs is much larger than the corresponding ratio for solar-type stars… The absolute numbers of detections are similar, but the number of surveyed solar-type stars is an order of magnitude larger. This may be an observational bias due to the lower mass of M-dwarf primaries, or truly reﬂects more frequent formation of Neptune-mass planets around M dwarfs. The factual conclusion remains that Neptune-mass planets are easier to ﬁnd around M dwarfs.

The work of Laughlin and others continues to suggest that lower mass stars like M dwarfs should produce low mass planets, which accounts for the presence of the Neptune-class and smaller worlds we’re discussing (this trend should also hold for solar mass stars that emerged from metal-poor nebulae). The new finds around Gliese 581 help to bolster these trends, while making it clear that finding more low-mass planets like this one will help firm up our theories. Obviously, all this plays into the building of target lists for future space-borne missions that will look for transits and (later) do spectroscopic analysis of planetary atmospheres.

For those of you in the UK, I’ll be discussing the Gliese 581 find on BBC radio some time between 1700 and 1800 BST today.

This seems to confirm (more or less) what I suggested in my comment under the previous post: that (low-metallicity) M-dwarfs have relative ‘mini-giant’ planets, i.e. Neptune/Uranus class subgiants (and even smaller) instead of Saturn/Jupiter giants. And in closer orbit. The EPE has 3 good stellar examples of this (with total of 5 planets).
Remains my question: what about medium-high metallicity M-stars? ‘Normal’ planetary systems with small rocky planets in close orbit and gas giants on the outside, or giant planets in close orbit? The known sample seems very small, but Gl 876 with ‘near-normal’ metallicity (about 3/4 of solar) has besides a subgiant in very close orbit also a ‘Saturn-class’ in close orbit (0.13 AU) and a (above) ‘Jupiter-class’ in close orbit (0.21 AU).
And there is at least one M-dwarf with solar metallicity (HD 41004 B) that has a supergiant planet (may even be a brown dwarf at 18 MJ) in very close orbit.
Gj 849 with metallicity almost 1 and a half times solar has a (almost) Jupiter-class planet at asteroid-belt-like orbit (2.35 AU).
So, though the sample is very small a tentative picture seems to emerge for M-class stars: low-metallicity (this is clearest and most consistent) have completely miniaturized planetary systems with subgiants in close orbit. As metallicity increases (but this is uncertain) the planets become bigger, but what happens to the orbits? Gl 876 and Gj 849 (and HD 41004 B ?) seem to suggest still relatively small orbits.

How can a planet be considered habitable if it’s got about twice the gravity of Earth? I don’t think I’d enjoy walking around weighing 531 pounds.

(Here’s a link to calculate your weight in pounds on the surface of this planet. You just need to enter your mass in kg into the equation, since it’s set for my 113kg: http://preview.tinyurl.com/39dehk )

Hello, my name is Ganzog the Great, and I reside on a planet called Gliese 581c.

Our recent discovery of a possibly habitable planet orbiting a bright yellow star called “the Sun” has erupted in controversy. While it may seem optimistic that life can evolve on such a planet, we find it unlikely that intelligent life could evolve on a world where day and night are not constant, like they are on our precious world. Imagine: having to evolve ways to cope with both day AND night!

And the new planet’s gravity? Hah! At half the strength of ours, life would no doubt fling itself into space before it could evolve a way to stay fastened to the ground in such puny gravity. And what a hot, massive star they’ve got over there, if they even exist! It’ll be a glowing red giant in a few short billions of whatever passes for its years.

Next you’ll be telling us that they reproduce sexually, instead of the way the Universe intended: filling out the proper forms and oozing around in line.

Actually, Ganzog, if you’ll look at the research conducted by Jor-El of the Kryptonian Science Council, you’ll find it argued that the greater energy present in yellow-star light, combined with the lower gravity of a planet like Sol e (the first rocky planet detected in that system, after the four giant planets), might actually enable the inhabitants of more massive planets around red stars to gain supernormal physical and sensory abilities.

Unfortunately, further reports from Krypton inexplicably ceased immediately after Jor-El submitted his research brief on a prototype interstellar probe employing breakthrough propulsion technology. We suspect this may be due to the loss of Science Council funding for the Kryptonian Institute for Advanced Concepts.

Life forms on such a tide-locked planet would likely be restricted to the day side. I also suspect that the band separating dayside from nightside would be stormy and perpetually overcast. Perhaps any intelligent beings there have never SEEN the stars. (Except perhaps for a few intrepid explorer-types who did not freeze on expeditions to the night side: “Over there, the sky is black and there’s all these white flecks in it!” “Yeah, sure! And so what?”)

Congratulations on your post a few months back talking about if you were a betting man you would say that within the next ten years a potentially habitable planet would be discovered around a red dwarf. However you were only wrong on one thing–the method of discovery. Radial velocity my friend. Still, WOW! GREAT JOB ON THAT HUNCH!

This (great) article and the two I quoted above do bring to my mind another potential spoiler for terrestrial planets around M-dwarfs: if indeed the common thing for M-stars is to have one or more Neptune class *subgiants in close orbit* (emphasis intended), then what are chance for a terestrial planet in a stable equally close orbit? I mean the concept of ‘forbidden zone’, where the large planet sucks up planetary matter during planet formation ánd gravitationally perturbs any terrestrial planet formation nearby. Of course the mass of a Neptune/Uranus is nothing like a Jupiter, but the orbits and hence proximities are also much closer.

Imagine having 2 or 3 Neptunes/Uranuses well within the orbit of Mercury. Would there still be a chance for an earthlike planet in close orbit? Surely computer models can work this out, or maybe have already done so.

djlactin said: “Life forms on such a tide-locked planet would likely be restricted to the day side.”

Surface life, perhaps. But the research suggests that the oceans could remain liquid on the far side. And who knows? There are life forms on Earth that don’t rely on sunlight. Deep-sea organisms rely on geothermal energy. Some organisms in caves never see the sun, and rely entirely on sustenance brought by other organisms from the outside. Similarly, there could be a life cycle on a tide-locked whereby nightside organisms gain nourishment from animals that travel there from the dayside, whether crossing the terminator under their own power or washing downstream or whatever.

There’s also the fact that nightside life would be more protected from flares. Although it’s possible that dayside life could evolve radiation resistance like the extremophile Deinococcus radiodurans:

marsbound, thanks for those kind words about my hunch! And you’re exactly right — radial velocity was the method used rather than transits. In fact, HARPS seems unbeatable at radial velocity work. What an accomplishment, and there is the promise of finding even smaller worlds with these methods.

If the orbit were elliptical, there would be wobble along the twilight zone of the planet. One consideration I’ve not seen considered is the evolution of the planet itself, and the impact that would have on developing life. Scientists have speculated about a migration from Gliese 581’s outer reaches.

A planet massing five earths would likely have internal heating mechanisms, possibly in excess of Earth’s. Add to that the relative proximity of two Neptune-sized planets–and remember we’re talking not hundreds of millions of miles, but mere millions. Though small, Europa and Ganymede are close enough to turn Io into a sulfuric hell. What would be the effect of nearby Neptunes?

This new planet strikes me as more likely to be an extreme environment for the possibility of life, more like Mars than Earth.

That said, the ESO people have produced a stunning achievement in technology and it seems clear that we’ll have even more attractive planet possibilities to ponder and speculate about within a short time.

Aside from the obvious, why are we getting so excited about
an exoplanet that is similar to Earth? Shouldn’t we be getting
more excited about worlds that aren’t anything like ours? Why
bother exploring space if we are only looking for copies of
ourselves?

Good question about moons. The Hill sphere for Gliese 581 c has a radius of 278,000 km (taking a mass of 5.1 earth masses for the planet, and 0.31 solar masses for the star). As a general rule, moons are stable out to about one third of the Hill radius, putting the outermost stable moon orbit at 93,000 km. If we assume a density similar to Earth for the planet (this is an underestimate for a terrestrial planet of this mass, but if we’re taking this to be some kind of ocean planet, supercritical or not, it will be larger than a rockball), this gives a planetary radius of 11,000 km. The Roche limit is then at 27,000 km for objects of similar density, further out for objects less dense. This gives a fairly narrow zone for satellites to exist (27,000-93,000 km).

However, the planet is likely tidally locked, which means all satellites are going to be sub-geostationary. Therefore tidal effects would gradually cause their orbital radii to decrease. Combine that with the narrowness of the zone of stable orbits, and I’d guess anything substantial (it turns out large moons migrate faster) would have been destroyed long ago.

I wonder if a periodic rotation of the planet and/or the crust might occur if the planet has significant amounts of water on the surface?

Could the ice cap on the dark side grow heavy enough that it might throw off the equilibrium of the planet just enough to cause it to be rotated? Could this pattern of melting, freezing, and rotation become a continuous cycle?

“Aside from the obvious, why are we getting so excited about
an exoplanet that is similar to Earth? Shouldn’t we be getting
more excited about worlds that aren’t anything like ours? Why
bother exploring space if we are only looking for copies of
ourselves?”

We’re looking for whatever we can find. I’m sure scientists would be delighted to find life that’s radically different from us — but how would we know what to look for? We’re just at the beginning of this process, so it makes sense to start with the familiar, the stuff we know how to test. Just because we’re taking it one step at a time doesn’t mean we intend to stop at the first step.

Besides, we’ve already found hundreds of worlds that aren’t anything like ours. This is the first one we’ve found that’s even close. And that’s what makes it important. Just the knowledge that our planet isn’t unique in the universe is significant.

Our planet is unique: it’s our home. We evolved here, we live and die here. For us, Earth is the most important place in the universe.

Turning to Giliese, which I am calling Udria (after her discoverer), anyone know when info about atmospheric composition will be in? I’m not that familiar with the process, but if they’re using spectroscopy analysis shouldn’t take too long with the computers they’ve got.

I personally think that anywhere in the universe life can exist it will.

Abstract: This Letter reports on the detection of two super-Earth planets in the Gl581 system, already known to harbour a hot Neptune. One of the planets has a mass of 5 M_Earth and resides at the “warm” edge of the habitable zone of the star. It is thus the known exoplanet which most resembles our own Earth. The other planet has a 7.7 M_Earth mass and orbits at 0.25 AU from the star, close to the “cold” edge of the habitable zone. These two new light planets around an M3 dwarf further confirm the formerly tentative statistical trend for i) many more very low-mass planets being found around M dwarfs than around solar-type stars and ii) low-mass planets outnumbering Jovian planets around M dwarfs.

Rebecca, many methods have been suggested for interstellar flight, from antimatter to various kinds of fusion. One I’ve always been interested in is the interstellar lightsail. Here’s an earlier article I did on these:

My questions are simple, I’m no science expert but you guys seem pretty adept at this and I was wondering if any of you knew any more info about this.

basically I wanna know just how far our eyes stretch, meaning we detected the prescene of this this planet via some ground based telescope correct? Currently I’m sure were not capable of launching a satellite that can give us video feed, or snap pictures of a planet that far away, but do we have any ability to detect if there are any orbital machinations surrounding Giliese 581? Meaning if we could detect any satellites surrounding that planet or anything remotely techinologically advanced, we could get better insight into what the planet hosts. Technology like that would indicate that the planet houses a species as advanced as we, or more….the possibliities are limitless, haha, I may be getting carried away here…but damn, this is a discovery for the ages. Just think, let’s assume that there are a species there on par with us, and as we discovered this planet…there discovering ours. Were one step closer to encountering extraterrestial life, for better or for worse. We can’t remain alone forever, the galaxy is a never ending realm, there is life elsewhere, we can’t be so naive as to think were the only ones, contact will be made, and I’m no sci-fi nerd believe me, but it’s highly plausible…here’s proof.

Kevin, these are good questions. Right now we don’t have the ability to get an actual image of a planet like this. Gliese 581 c was found using so-called ‘radial velocity’ methods, meaning the scientists involved measured the tiny movements of the star as it was influenced by the gravity of its planetary system. This can be an extremely effective way to find planets, but it doesn’t yield images. That will have to wait for space-based telescopes, but their day is coming. Already the European COROT spacecraft has found its first planet by using the ‘transit’ method — looking at the drop in light from the star as the planet moves between us and the stellar surface. COROT may be able to detect small, rocky planets like Earth, but still no images. The push to get actual images should yield results, but the time frame will depend on how we fund the missions that will be designed to produce them. Keep your eye on the Kepler mission:

As far as I know, both NASA and ESA have projects in the pipeline to accurately analyse the composition of exoplanet atmosphere within the next decade. However, due to budget cuts, the Nasa version – Terrestrial planet finder (don’t remember exact name) may not be realized.

>Christopher L. Bennett Says:
>April 26th, 2007 at 22:49

>“Aside from the obvious, why are we getting so excited about
>an exoplanet that is similar to Earth? Shouldn’t we be getting
>more excited about worlds that aren’t anything like ours? Why
>bother exploring

I agree but remember, this planet IS already pretty alien to ours no matter what. It’s light is mostly outside the visible spectrum and would probably seem to be in perpetual sunset/sunrise. The sun would be a huge red disk in the sky that you could likely stare at without looking away. Human life would have to be physically different (higher blood pressure? etc.) to survive there. Who knows, intelligent life it existed there could be completely different.

As for exoplanets that are unusual, the very first one ever discovered matches that. Google for PSR B1257+12 and its planets.

As regards future planet discovery, it really can only get better. The only reason so many hot Jupiter exoplanets were discovered early on was because
they are so big and so near their parent stars that they have a much greater pull on the star hence creating a much more readily detectable doppler shift/radial velocity.

HARPS allows unprecedented accuracy in the region of 2 Earth masses. But thats still not enough. The only way we can detect far away Earth mass and less worlds right now is by using a different technique (only valid for detecting planets around pulsars I believe) like that used to detect the planets of PSR B1257+12.

Anyway, I think it’s all very exciting. It seems that smaller rocky or water planets are much more common around M class red dwarfs and we’ll soon be able to detect all of them that are there in our galactic neighbourhood.

We (hopefully) will die knowing we found out if other inhabitable/inhabited! planets exist in our galactic neighbourhood.

Promoton, while the original TPF mission is indeed on indefinite hold, the research into technologies for this class of mission continues. Starshades are under active study, and new coronagraph technologies keep that idea alive as well. So while we won’t get the TPF that the Jet Propulsion Laboratory had in mind a few years ago, we’ll one day get a planet-finder mission that will chart much of the same course. Whether they’ll call it Terrestrial Planet Finder then is anyone’s guess.

Gliese 581c is within about 6 million miles of it’s star and is probably tidally locked with it. It also may have a thick atmosphere and a very deep ocean. If animal life doesn’t exist there, I’d sure like to know what does. The tidal pull from the hot Jupiter which orbits inside it’s orbit should be intense and could spark alot of tectonic activity. This planet should be intensely scrutinized for as long as it takes to find these answers. It’s star will burn on the main sequence for a virtual eternity compared to ours so time for evolution exists there in spades.

Charter

In Centauri Dreams, Paul Gilster looks at peer-reviewed research on deep space exploration, with an eye toward interstellar possibilities. For the last seven years, this site has coordinated its efforts with the Tau Zero Foundation, and now serves as the Foundation's news forum. In the logo above, the leftmost star is Alpha Centauri, a triple system closer than any other star, and a primary target for early interstellar probes. To its right is Beta Centauri (not a part of the Alpha Centauri system), with Beta, Gamma, Delta and Epsilon Crucis, stars in the Southern Cross, visible at the far right (image: Marco Lorenzi).

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